1. Field of the Invention
The present invention generally relates to the field of optical fibers, and to manufacturing methods thereof. More particularly, the present invention concerns a method of making optical fibers featuring a low polarization mode dispersion.
2. Description of the Related Art
Optical signals transmitted through single-mode optical fibers comprise two orthogonal polarization modes, usually denoted Transverse Electric (TE), and Transverse Magnetic (TM). In case the fiber has a perfectly cylindrical core of uniform diameter, the two TE and TM modes propagate at a common velocity (i.e., they have same propagation constants β1=β2). However, in real-life optical fibers the cylindrical symmetry of the fiber core may be disrupted due to shape defects or non-uniform stresses. As a result, the refractive index becomes dependent on the polarization state, and the two TE and TM modes exhibit different propagation constants β1≠β2. A phase difference builds up between the two polarization modes as they propagate along the fiber, and the fiber is said to exhibit “birefringence” (or, equivalently, the fiber is said “birefringent”). An indication of the fiber birefringence is provided by the absolute value difference Δβ=|β1−β2| between the two propagation constants β1 and β2 for the two TE and TM modes. In particular, the birefringence introduced by shape and stress asymmetry is known as “intrinsic birefringence”.
The structural and geometrical irregularities of the optical fiber that give rise to birefringence typically originate from the fiber preform itself, and are modified during the process of drawing the fiber. This process is usually carried out by means of an apparatus known as a “drawing tower”, starting from a glass preform. In practice, after the preform has been placed in vertical position and heated to a temperature above the softening point within a suitable furnace, the molten material is drawn downwards at a controlled speed, in such a way as to produce a threadlike element that forms the optical fiber. In this process, asymmetrical stresses are typically applied to the fiber.
In a birefringent fiber, the two components TE and TM of the fundamental optical mode propagating through the fiber, assumed to be initially in phase with each other, return to be in phase again only after a certain propagation length, commonly known as the “beat length” (LB). In other words, the beat length indicates the period of repetition of a certain state of polarization (on the assumption that the fiber maintains a constant birefringence over this length). The beat length LB depends on the birefringence, and in particular it is: LB=2π/Δβ. Therefore, the higher the birefringence, the shorter the beat length.
Apart from a restricted class of fibers, known as “polarization-preserving” or “polarization-maintaining” optical fibers, in which asymmetry is deliberately introduced to generate birefringence, birefringence is normally detrimental to the optical fiber performance.
In fact, when pulsed signals are transmitted through an optical fiber, birefringence is a cause of pulse spreading, since the two polarization components, TE and TM, of the signal travel at different group velocities, and become dispersed. This phenomenon, known as “Polarization Mode Dispersion” (shortly, PMD), has been widely studied in recent years because of its importance in periodically amplified light guide systems.
Typically, the PMD phenomenon leads to a limitation of the width of the signal transmission band and, consequently, a degradation of the performance of the optical fibers along which the aforesaid signals are transmitted. This phenomenon is therefore undesirable in systems of signal transmission along optical fibers, especially in those operating over long distances, in which it is necessary to minimize any form of attenuation or dispersion of the signals to guarantee high performances in transmission and reception.
A known way to produce optical fibers with reduced PMD is to include a fiber spinning step during the fiber drawing stage. For the purposes of the present description, the term “spin” identifies a torsion that is frozen-in during the fiber drawing, being applied to a viscous portion of the fiber and kept as a structural modification of the fiber after cooling.
The benefits deriving from spinning the fiber during drawing are for example described in the U.K. patent application GB-A-2101762: in that document, it is discussed that spinning is performed at a relatively high rate, so that its spatial repetition frequency, or spin period, is small compared to the fiber beat length LB; as a result, a “spun” optical fiber features a reduced contribution of birefringence due to form and stress asymmetries.
Due to spinning, the fiber under drawing undergoes a rotation of its polarization axes. As a result, when optical pulses are transmitted into the optical fiber, they propagate alternately on the slow and fast birefringence axes, thus compensating the relative propagation delay and reducing the pulse spreading. This is qualitatively equivalent to having a local effective refractive index for the optical pulses equal to the average refractive index on the two axes, the average being taken over the pulse length along the fiber. Theoretical studies have shown that the dominant process for the reduction of PMD in a spun fiber is the averaging of the local fiber anisotropy by the rapid procession of the axes of asymmetry along the fiber.
Several spin functions have been proposed in the art. For example, in the above cited U.K. patent application GB-A-2,101,762 it is stated that the preform may be spun at a substantially constant rate, but it could even reverse in direction, oscillating from a right-handed to a left-handed twist. The U.S. Pat. No. 4,504,300 addresses drawbacks related to rotation of the preform, and proposes a spinning technique consisting in rotating the fiber, instead of the preform, during fiber drawing. The U.S. Pat. No. 5,418,881 proposes to impress the spin to the fiber alternately clockwise and counter-clockwise direction. Alternate spinning is also proposed in the U.S. patent application US2001/0020374, as preferred to unidirectional spinning, since it prevents the presence of residual fiber torsions (i.e., of residual fiber twists) on the fibers wound onto the collecting spool, thus making easier both the unwinding and wiring operations of the same.
In the published International application WO 2004/058654, a method is described wherein a substantially sinusoidal spin is applied to an optical fiber while drawing it. The spin function frequency, the length of the viscous zone of the fiber being drawn, and the drawing speed are such that each optical fiber portion, while being in the viscous state, undergoes a torsion and at least 50% de-torsion. In this way, the amplitude of the frozen-in spin function (i.e., the torsion permanently impressed on the fiber, when cooled, during the spinning process, as a result of the torsional deformation undergone by the viscous zone of the fiber material in the drawing furnace) is much lower than the amplitude of the actually imparted spin function (i.e. the torsion effectively applied to the fiber during the drawing process; the actually imparted spin function corresponds to the spin applied to the fiber by a spinning apparatus, less mechanical effects like slippage at the interface between the fiber and the spinning apparatus); however, despite this, a significant PMD reduction is achieved.
In WO 2004/058654 it is pointed out that experiments revealed a significant difference between the torsion applied to the fiber during drawing and the frozen-in torsion; in particular, the difference between the applied torsion and the frozen-in torsion (both expressed in turns per meter) is very small at low values of the spin function frequency, while it increases with the increase in the spin function frequency; in other words, the transfer of a spin function to the fiber depends on the spin function period: the longer the spin function period, the higher the transferred spin amplitude, with a different maximum at a certain period value. In particular, referring to FIGS. 4 and 5 of the cited application, and denoting ν the spin function frequency, L the viscous zone length, V the drawing speed, and k a dimensionless parameter equal to ratio ρVL/μ, being ρ the density of the fiber material and μ the viscosity thereof in the viscous zone, the lines labeled “k<∞” show that the difference between the applied torsion and the frozen-in torsion is substantially null for small νL/V values, while it increases with increasing νL/V values, with a difference maximum at a certain νL/V value.
In the published International application WO 2002/03115 it is disclosed that spin functions can be optimized to reduce the PMD, depending on the fiber beat length. In particular, if the spin period is longer than the beat length, the spin function is optimized only for that beat length. On the contrary, if the spin period is shorter than the beat length, the spin function remains the optimum one also for different beat lengths. Since the fiber beat length is in general not known before drawing the fiber, and it varies along the fiber, optimized spin functions with short spin periods are preferred. In particular, from WO 2002/03115 it can be deduced that the beat length of an optical fiber is not affected by the spinning process, as it depends only on the birefringence of the fiber (in that document there is stated that in a commercial production of optical fibers of the same type, i.e. having substantially the same refractive index profile and made by the same production process, an expected beat length can be generally determined, before the drawing of the optical fiber, in a statistical way, or by DGD—Differential Group Delay—measurements on the unspun fiber). WO 2002/03115 also states that advantageously the period p of the spin function may be chosen so as to be lower than the expected fiber beat length: in such a case, the inventors have found that the variability with respect to the fiber beat length of the spin function parameters useful for obtaining a substantially periodic DGD is further reduced. Generally speaking, the optimum spin depends on the fiber beat length. However, under the short period assumption the solutions are independent on the beat length.
In S. M. Pietralunga, M. Ferrario, P. Martinelli, M. Martinelli, “Direct Observation of Local Birefringence and Axis Rotation in Spun Fiber With Centimetric Resolution”, IEEE Photonics Technology Letters, Vol. 16, No. 1, January 2004, pp. 212-214, a measurement method is described that allows the direct observation of the rotation of the local linear birefringence axes along a spun fiber, and brings into evidence the periodic behavior of the birefringence retardation, as theoretically forecasted, along fibers spun with a constant spinning rate during the drawing stage.
In Y. Wang and C.-Q. Xu, “Characterization of spun fibers with millimeter spin periods”, Optic Express, 16 May 2005, Vol. 13, No. 10, pp. 3841-3851, a method is proposed to precisely measure short spin periods (of the order of a few millimeters) with a spatial resolution of 0.1 mm. Constant (i.e., unidirectional) spin is considered.
In M. Ferrario, S. M. Pietralunga, R. Bratovich, and M. Martinelli, “Alternate Spin Profile Reconstruction in Low-PMD Fibers”, JThE52, 2005 Quantum Electronics and Laser Science Conference (QELS), pp. 1714-1716, a measurement technique is described according to which any kind of spin profile, like a sinusoidal spin profile, transferred into fibers during the drawing process is accurately determined by exploiting a cut-back method together with a deterministic waveplate model.